1. Field of the Invention
The present invention relates to a solar cell with a semiconductor substrate whose surface is formed with microscopic protrusions and recesses for reducing reflection of light, a manufacturing method thereof, and an electrode material used for the solar cell.
2. Description of the Related Art
Solar cells include solar cells using monocrystal line semiconductor substrates, those using polycrystalline semiconductor substrates, and those using amorphous semiconductor substrates. In view of mass production productivity, the best of these are the solar cells using polycrystalline semiconductor substrates.
In order to improve the efficiency of such solar cells using polycrystalline semiconductor substrates, various approaches have been made.
One example of these is forming microscopic protrusions and recesses on the surface of the semiconductor substrate so as to introduce as much light incident on the solar cell as possible into the semiconductor substrate, and to trap as much light introduced into the semiconductor substrate as possible within the semiconductor substrate.
Such microscopic protrusions and recesses are formed, for example, by the Reactive Ion Etching: RIE method. By this method, it is possible to form uniform protrusions and recesses on the surface and reduce surface reflection more effectively even when polycrystalline silicon is used for the semiconductor substrate, irrespective of the irregular plane directions of the crystals.
(A) However, since such microscopic protrusions and recesses formed on the surface of the silicon substrate have heights of as small as 2 μm or less and pitches between protrusions and recesses of as small as 0.2 to 2.0 μm, they are susceptible to impact and stress.
Accordingly, when conductive paste such as silver paste is applied by printing followed by firing to such microscopic protrusions and recesses formed by the above method, if the silver paste fills the bottom portions of the microscopic recesses without allowing clearance, the microscopic protrusions and recesses collapse due to stress during the firing, resulting in generation of a great number of defects such as micro cracks in the bottom portions. The electric properties are therefore deteriorated. When this kind of defects are generated in areas below an electrode, occurrence of recombination of carriers increases, causing the electric properties, in particular, the open circuit voltage to drop.
(B) Meanwhile, there has been a process in which a silicon substrate is provided with a diffusion layer containing an opposite conductivity type impurity, over which a silicon nitride film, silicon dioxide or titanium dioxide film or the like is formed as an antireflective film.
A process has been proposed for formation of an electrode on this antireflective film, in which partial removal of the antireflective film is not performed and an electrode material in paste form is printed over the antireflective film and then fired as it is.
That is, by fusing by heat the paste-form electrode material applied by printing to the antireflective film simultaneously with fusing the material of the antireflective film located beneath the electrode material, the electrode material and the silicon substrate are bonded together so that an ohmic contact is accomplished between the electrode material and silicon (fire through method).
For this electrode material, for instance, an electrode material in paste form composed of 100 weight parts of silver powder with particle sizes of about 0.1-2 μm, 10-30 weight parts of an organic vehicle and 0.1-5 weight parts of glass frit is used.
However, when this method is employed in a solar cell having a diffusion layer containing an opposite conductivity type impurity which is formed on microscopic protrusions and recesses on a silicon substrate to a thickness of as thin as 0.2-0.5 μm, it is difficult to stably fuse the material of the antireflective film, and as a result, there could be cases where semiconductor junctions between the shallow diffusion layer and the silicon substrate are destroyed.
In such a case, the fill factor of the solar cell characteristics is lowered, and accordingly output power of the solar cell is reduced.
Even when lowering in the fill factor of the solar cell characteristics does not arise, the bonding strength between the semiconductor substrate and the electrode is so poor that inconveniences arise in the assembly and testing process.
(C) It has been known that when a solar cell is produced by forming a diffusion layer containing an opposite conductivity type impurity on the surface of a silicon substrate, forming an antireflective film which also functions as a passivation film, over the diffusion layer, and forming an electrode on top of them, shallowing the depth of the impurity diffused on the surface of the silicon substrate causes the sheet resistance to increase so that a great deal of current and high voltage can be obtained.
However, it becomes difficult to fuse the material of the antireflective film without destroying the semiconductor junctions so as to establish good contact between the electrode and silicon.
It is an object of this invention to provide a solar cell in which the conventional problem in that electrode material, silver, is trapped in the recessed areas of the microscopic protrusions and recesses and causes defects to be generated in the microscopic protrusions and recesses located below the electrode due to stress generated during firing of the electrode has been eliminated, a manufacturing method thereof, and an electrode material for use therein.
It is another object of this invention to provide a solar cell having both excellent characteristics and electrode strength by reducing the surface reflectance and without requiring a complicated process.